1. Field
This invention relates to furnaces of the type which provide a heated active zone for inducing reactions in feed material conveyed through the zone. It is particularly directed to furnaces of this type in which the active zone is annular and substantially horizontal.
2. State of the Art
Reaction furnaces of various types are well known. They have long been used for removing moisture and other volatiles from contaminated or raw material feed stocks. They have also been used to alter the chemical composition of feed stocks or to effect the chemical reaction or breakdown of constituents in the feed. In any event, such furnaces, or kilns, conventionally comprise a chamber within a housing or structural support, means for heating all or a portion of the chamber and means for moving material through the heated, or active, zone of the chamber.
An exemplary type of reaction furnace is the rotary kiln. Such kilns have found broad application in the chemical and minerals processing industries. In many applications, such as the regeneration (or reactivation) of carbon, kilns are generally not process sensitive; that is, they are capable of regenerating carbon from a variety of sources without regard to variations in moisture content or the presence of fouling contaminants, such as flotation reagents or lime. Kilns can be constructed to vent volatiles and steam from the vicinity of the reaction zone. They are capable of operation whether feed is present in the active zone or not. All of these features are advantageous, but rotary kilns nevertheless suffer from certain disadvantages and limitations.
A rotary kiln comprises a cylindrical barrel which is heated to a high temperature and rotated for prolonged periods between supports. The barrel is only partially filled with feed material The material is dynamically mixed as it travels from the feed end to the discharge end of the barrel. The barrel length must be substantial, generally no less than twelve feet, to provide adequate residence time within the active zone and adequate capacity. For large capacity operations, kiln barrels as long as forty (40) feet and having diameters of four feet or more are not uncommon. The natural tendency of the barrel to sag is increased by the elevated temperature of operation typically 1200.degree. to 1500.degree. F. As the barrel rotates, reverse bending inevitably occurs, inducing high stress and ultimate structural failure. Construction of a rotary kiln with a reasonable life expectancy is thus very expensive. To the extent that economies of construction are attempted, the reduced quality and/or quantity of machined parts leads to additional stress-related problems; e.g. shock. Increasing the strength (and thus the weight) of the barrel raises the cost of construction inordinately for most applications, because all of the ancillary components required to support and drive the barrel must also be increased in size and/or number.
Another type of reaction furnace which has gained commercial success for a variety of applications is the vertical kiln in which the active zone comprises a plurality of tubes or an annulus between concentric cylinders. The active zone is disposed approximately vertically and is entirely filled with material during operation. Feed material is introduced at the top of the zone and migrates downward under the influence of gravity. The zone is thus static and avoids many of the stress-related problems associated with rotary kilns. Of course, the static zone cannot provide the dynamic mixing characteristic of a rotary kiln. Vertical kilns have the advantage of comparatively low cost construction, even from high quality materials, and require relatively little installation space. They are practical for on-site installations in situations which would not justify the installation of a rotary kiln. Vertical furnaces, however, also suffer from certain limitations and disadvantages.
The temperature gradient from the bottom to the top of the active zone in a vertical furnace is typically substantial. Feed enters the top of the zone carrying moisture and volatiles. Steam and other gases are thus driven from the feed as it migrates downward and gains heat energy. Inevitably, volatiles and steam rising from the lower portion of the active zone (which is of relatively high temperature) tend to reflux (condense) as they enter a cooler upper region of the active zone. These refluxed materials may become sufficiently concentrated to eventually pass out the discharge end of the active zone. In any event, the capacity of the furnace is negatively impacted by the necessity for revolitilization of condensed materials. Another significant problem encountered with vertical furnaces is the tendency of feed material to become confined, sometimes compacted, by virtue of the relatively limited cross-sectional area of the active zone. Flashing or blow-back of feed from the furnace is thus possible, especially if the porosity of the feed material is reduced by compacting or refluxing of volatiles.
The regeneration of activated carbon used in a variety of chemical, mineral processing and water treatment applications is of increasing importance. While the rotary kilns and vertical furnaces heretofore available can be used for that purpose, they have not been entirely satisfactory. The consumers of reactivated carbon are typically not economically structured to acquire and operate a rotary kiln. It has thus become the practice for many such consumers to arrange for spent carbon to be hauled to and from a rotary kiln owned by another for processing. The scale of operation of the kiln is often such that the consumer cannot be guaranteed return of the specific carbon sent out for processing. Thus, each consumer risks receiving reactivated carbon with unfamiliar or hazardous contaminants. Moreover, contract reactivation of this type has been very expensive and has customarily imposed a kiln loss of between ten to fifteen percent on the consumer. The use of vertical furnaces on site, while more economical and while imposing a kiln loss of typically about five percent (5%), requires operational and maintenance expertise. The refluxing and blowback tendencies of presently available vertical kilns have discouraged their use despite their inherent economic and processing advantages.
There remains a need for an improved reaction furnace which offers the advantages of a rotary kiln but also offers the low cost and low space requirements of a vertical furnace without the attendant disadvantages of refluxing and blowback inherent in such furnaces. Such an improved reaction furnace would find use in many applications currently served by existing types, but it would find specific application in on-site installations for the regeneration of carbon.